Bisphenol A has been used to prepare polymers having a variety of properties and uses. For example, bisphenol A may be reacted with epichlorohydrin to provide polymers useful in packaging coatings. There is a desire to reduce or eliminate the use of certain bisphenol A-derived polymers in food or beverage container coatings. Although a number of replacement coating compositions made without bisphenol A have been proposed, some replacement compositions have exhibited insufficient coating properties such as insufficient corrosion resistance on metal substrates, insufficient flexibility or insufficient toughness.
In addition, in recent years styrene has also come under greater scrutiny. Although the balance of scientific evidence indicates that coatings containing polymerized styrene are safe for food-contact end uses, there is a desire by some to eliminate styrene from such end uses. Styrene, however, brings advantageous properties that contribute to the overall performance of food or beverage can coatings and can be difficult to replicate using other materials. As such, the use of styrene in conventional such coatings has been commonplace.
The balance of coating performance attributes required for a coating composition to be suitable for use as a food or beverage can coatings are particularly stringent and are unique from other coating end uses. As such, coatings designed for other ends uses are not typically suitable for use as food or beverage can coatings.
For example, coatings for use on food or beverage containers should avoid unsuitably altering the taste of the packaged food or beverage products, and should also avoid flaking or chipping into the packaged products. The coatings should also resist chemically aggressive food or beverage products (which can have a complex chemical profile, including salt, acids, sugars, fats, etc.) for extended periods of time (e.g., years). Food or beverage container coatings should also have good adhesion to the underlying substrate and remain sufficiently flexible after curing, because subsequent fabrication and denting during transportation, storage or use (e.g., by dropping) may cause the metal substrate to deform, which will cause the coating to flex. A brittle coating will crack during flexure, exposing the container metal to the packaged products, which can sometimes cause a leak in the container. Even a low probability of coating failure may cause a significant number of containers to leak, given the high number of food and beverage containers produced.
Accordingly, it will be appreciated that what is needed in the art are improved coating compositions that are made without intentionally using bisphenol A and/or styrene, but which exhibit the stringent balance of coating properties to permit the use of such coating compositions on food or beverage containers.